How come it says the popularity of Ruby has plummeted?
According to the article they're basing it on job postings and stack overflow questions. Doesn't seem like a great methodology but I can't think of a better one.
StackOverflow already covers a great part of the possible (most common) Ruby related questions. I'm not surprised there aren't so much new ones in the last 1-3 years.
About job postings, I don't know, I still get an average of 3-5 ruby job opportunities per week from recruiters. The same goes for several ruby devs I know.
On the other hand, I see a lot of people switching to other new languages/stacks for several reasons (some of which are mentioned in the article). I'm one of them, but I still use ruby when it might fit the job (or is a requirement).